Hydrophilic polymeric articles have been used as water-spreading materials. The hydrophilic nature of the polymer causes water droplets to spread out over the surface of the polymer as a thin layer. This water-spreading property can be very important in certain instances, particularly on a traffic sign that employs a retroreflective sheeting.
Retroreflective sheetings have the ability to return a substantial portion of incident light in the direction from which the light originated. This unique ability has made retroreflective sheetings commonplace on traffic signs. At nighttime, light from motor vehicle headlights is retroreflected by the sheeting to display information on the sign to motor vehicle drivers.
Frequently, light transmission to and from a retroreflective sheeting is impaired by precipitation. A prominent form of precipitation that affects light transmission is dew formation, dew formation can be highly problematic because it occurs predominantly at nighttime when the retroreflective sheetings are operative.
When dew is present on a traffic sign in the form of small beaded water droplets, the path of incident and retroreflected light becomes significantly altered. This makes information on the sign much more difficult for passing motorists to read. In contrast, when the small water droplets spread out over the surface of the retroreflective traffic sign, the information on the sign is easier to read because the resulting thin layer of water does not disturb the path of the incident and retroreflected light to as great an extent.
To improve the performance of retroreflective traffic signs under dew conditions, investigators have developed hydrophilic surface coatings for retroreflective sheetings which allow water to spread out over the surface of the traffic sign. In U.S. Pat. Nos. 5,073,404, 4,844,976, and 4,755,425, for instance, T. Huang discloses a retroreflective sheeting that has a transparent coating comprising colloidal silica and a polymer selected from aliphatic polyurethanes, polyvinylchloride copolymers, and acrylic polymers. The colloidal silica is disposed in the polymer at about 10 to 80 weight percent (10 to 70 weight percent in the case of polyacrylates). The transparent coatings provide superior dew repellency, allowing the retroreflective sheeting to retain a higher percentage of its original brightness after being exposed to such moisture.
A number of other documents have disclosed hydrophilic materials; see, for example, U.S. Pat. Nos. 4,906,379, 4,576,864, 4,536,420, 4,478,909, and 4,409,285; U.K. Patent Application GB 2,249,041 A; and Japanese Patent Kokai H3-41402. The disclosures of each of these documents are briefly described below.
U.S. Pat. No. 4,906,379 to Hodgins et al. discloses a hydrophilic article that comprises a matrix containing molecules of a nitrile-containing polymer. Solely on the surface of the matrix, the polymer provides sufficient, uncharged, substituted amide groups to render the surface hydrophilic.
U.S. Pat. No. 4,576,864 to Krautter et al. discloses a water-spreading layer that is composed of colloid particles of metal or silicon oxide. The water-spreading layer is adhered to a plastic base by an adhesive comprising a non-water-soluble, organic solvent soluble and essentially non-swellable polar group containing polymer.
U.S. Pat. No. 4,536,420 to Rickert discloses a coating composition comprising a clear water-borne carboxylic acid functional polymer blended with an aqueous colloidal dispersion of a surface hydroxylated silica. The carboxylic acid functional polymer may be an acrylic resin. The composition produces a film when applied to a surface, and when hardened or cured, provides discrete canals in the film which tend to break up the water droplets and facilitate drainage of water from the surface.
U.S. Pat. No. 4,478,909 to Taniguchi et al. (corresponds to EP-A 0 051 405 A1) discloses an anti-fogging film comprising polyvinylalcohol, finely divided silica, an organic silicon compound and a hydrolysate thereof. The anti-fogging film is in the form of a cured film coated on a substrate, where the carbon/silicon weight ratio of the surface layer of the film is larger than that of the entire coating film and is not less than 1.7/1.0.
U.S. Pat. No. 4,409,285 to Swerdlow discloses a surface-modifying coating composition for reducing surface misting. The coating composition comprises an inorganic particulate component having an average particle size in the range of 7 to 50 nanometers, another inorganic particulate component having an average particle size in the range of 75 to 150 nanometers, polymer binder, and a volatile liquid medium. The inorganic particulate components may be colloidal silica and/or alumina particles.
U.K. Patent Application GB 2,249,041 A to the Imperial College of Science, Technology and Medicine discloses a modified hydrophobic plastic surface which has been subjected to an oxidation treatment and carries on the treated surface a layer of colloidal particles of hydrous metal oxide. The hydrous metal oxide causes the treated surface when wetted with water to demonstrate satisfactory non-fogging performance without loss of optical transmissibility.
Japanese Patent Kokai H3-41402 to Mitsubishi Rayon KK discloses a transparent, anti-dew material which is characterized in that a porous layer of silicon oxide with a thickness of 3,000 angstroms to 10 micrometers is formed on the surface of a transparent synthetic resin substrate. The silicon oxide layer has a preferred average grain diameter of below 2,000 angstroms. The porous silicon oxide layer can be formed by sputtering or ion implantation.